ARTICLE IN PRESS

Material Properties

Mechanical performance of coir fiber/polyester composites

S.N. Monteiro a, L.A.H. Terrones a, J.R.M. D’Almeida b,�

a Science and Technology Center, Universidade Estadual do Norte Fluminense, Av. Alberto Lamego, 2000-Horto-28015-820, Campos, RJ, Brazilb Materials Science and Metallurgy Department, Pontifıcia Universidade Catolica do Rio de Janeiro,

Rua Marques de Sao Vicente,225-Gavea-22453-900, Rio de Janeiro, RJ, Brazil

a r t i c l e i n f o

Article history:

Received 24 January 2008

Accepted 10 March 2008

Keywords:

Coir fiber

Composites

Flexural strength

Micromechanics

a b s t r a c t

The structural characteristics and mechanical properties of coir fiber/polyester compo-

sites were evaluated. The coir fibers were obtained from disregarded coconut shells that if

not properly processed constitute an environmental hazard. The as-received coir fiber was

characterized by scanning electron microscopy coupled with X-ray dispersion analysis.

Composites prepared with two molding pressures and with amounts of coir fiber up to

80 wt% were fabricated. Up to 50 wt% of fiber, rigid composites were obtained. For

amounts of fiber higher than this figure, the composites performed like more flexible

agglomerates. The results obtained for flexural strength allowed comparison of the

technical performance of the composites with other conventional materials.

& 2008 Elsevier Ltd. All rights reserved.

1. Introduction

Natural fibers such as cotton, flax and sisal have beenused since historical times in a large variety of products,ranging from clothes to house roofing. Today, these fibersare appraised as environmentally correct materials owingto their biodegradability and renewable characteristics.Moreover, lignocellulosic fibers are neutral with respect tothe emission of CO2 [1]. This is an extremely importantaspect, and puts lignocellulosic fibers as materials incontext with the Kyoto protocol.

In addition to plants that are cultivated with the mainpurpose of using the fiber, in other plants the fiber hassecondary or no commercial interest at all. This is the caseof the banana trees, which are cultivated for the fruits.From the leftover leaves and bark of banana trees, fiberswith good mechanical properties can be obtained [2].However, these fibers are seldom used since the treeis normally discarded as garbage after the fruits arecollected.

Another crop with a similar characteristic is coconut(Cocos nucifera). Plantations of coconut are spread all overthe world in tropical and sub-tropical regions, and are animportant item in the economy of many of these regions.The annual world production of coconut is about 42million MT, which would equate to almost 50 billioncoconuts [3]. The main use of coconut is for culinary pur-pose, and after extraction of the copra and/or of the liquidendosperm that fills the interior of the fruits the fruit shellis disregarded. Transformation industries and regionswhere consumption of coconut is high have, therefore, alarge problem to conveniently dispose of this waste, sincethe fruit shell has a long decay time. The coconut fruit is,in fact, adapted for being dispersed by seawater, and canfloat for months without rotting.

Growing attention is nowadays being paid to coconutfiber. Fibers extracted from the husk of the nut, known ascoir fiber, are now being commercially used, blended withnatural rubber latex in the production of seat cushionparts in automobiles [4]. These fibers are extracted fromthe external layer of the exocarp and from the endocarp ofthe fruit. The coconut palm can, in fact, be regarded as anintegral fiber producer because fibers can be extractedfrom many parts of the palm, such as from the leaf sheath,the bark of the petiole or from the midribs of leaves [5,6].

Contents lists available at ScienceDirect

journal homepage: www.elsevier.com/locate/polytest

Polymer Testing

0142-9418/$ - see front matter & 2008 Elsevier Ltd. All rights reserved.

doi:10.1016/j.polymertesting.2008.03.003

� Corresponding author. Tel.: +55 213527 1789; fax: +55 213527 1236.

E-mail address: dalmeida@dcmm.puc-rio.br (J.R.M. D’Almeida).

Polymer Testing 27 (2008) 591– 595

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Many aspects of the use of coir fibers as reinforcementin polymer–matrix composites are described in theliterature. Coir fiber–polyester composites were tested ashelmets, as roofing and post-boxes [7]. These composites,with coir loading ranging from 9 to 15 wt%, have a flexuralstrength of about 38 MPa. Coir–polyester composites withuntreated and treated (PMMA and PAN grafted) coir fibers,and with fiber loading of 17 wt%, were tested in tension,flexure and notched Izod impact [8]. The results obtainedwith the untreated fibers show clear signs of the presenceof a weak interface—long pulled-out fibers without anyresin adhered to the fibers—and low mechanical proper-ties were obtained. Although showing better mechanicalperformance, the composites with treated fibers present,however, only a moderate increase on the values of themechanical properties analyzed. Alkali treatment is alsoreported for coir fibers [9,10]. Treated fiber–polyestercomposites, with volume fraction ranging from 10% to30%, show better properties than composites withuntreated fibers, but the flexural strength of thesecomposites was consistently lower than that of the barematrix. A maximum value of 42.3 MPa is reported againsta value of 48.5 MPa for the neat polyester. Acetylation ofcoir fibers increases the hydrophobic behavior, increasesthe resistance to fungi attack and also increases the tensilestrength of coir–polyester composites [11,12]. However,the fiber loading has to be fairly high, 45 wt% or evenhigher, to attain a significant reinforcing effect when thecomposite is tested in tension. Moreover, even with highcoir fiber loading fractions, there is no improvement in theflexural strength [12]. From these results, it is apparentthat the usual fiber treatments reported so far did notsignificantly change the mechanical performance ofcoir–polyester composites.

Since most data in literature with few exceptions [12]usually cover only a specific loading fraction of fibers, andremembering that the increase of cost due to the treat-ment of the fibers must be a point of concern, this workwas aimed at analyzing the flexural mechanical behaviorof untreated coir–polyester composites covering a largerrange of weight fractions. The effect of the moldingpressure on the flexural strength of the composites wasalso evaluated.

2. Experimental procedures and materials

Individually loose coir fibers were used in two distinctforms: tangled or pressed in mats with a thickness of 1.0 cm.The fibers were used untreated, except that they were driedat 50 1C for 24 h. Fig. 1 illustrates the two kinds of coir fibers,tangled mass and pressed mats, used in this work.

A commercially available unsaturated orthoftalicpolyester resin with 1 wt% of methyl-ethyl-ketone ascatalyst was used as matrix for the composites. Afterbeing thoroughly mixed, the resin was poured into thecavity of a steel mold, which was previously filled with asuitable amount of coir fiber. Composites with amounts ofcoir fibers ranging from 10 to 80 wt% were manufacturedat two pressure levels, namely: 2.6 and 5.2 MPa. The curewas done under pressure at room temperature.

As the fibers in any of the two configurations (tangledor mat) did not have a preferred orientation, thecomposites fabricated in the present work are consideredas randomly oriented. Rectangular specimens 122 mmlong, 25 mm wide and 10 mm thick were bend tested,using the three-point bending procedure, on a 100 kNcapacity testing machine. The velocity of the test was5 mm/min, which corresponds to a strain rate of 1.6�10�2

s�1. The span-to-depth ratio was maintained constant at 9,and the minimum number of specimens used for each ofthe test conditions and coir fiber arrangements was 6.

Before their incorporation in the composites, the coirfibers were analyzed by scanning electron microscopy(SEM). The analysis was performed on gold-sputteredsamples in a microscope, coupled with EDS, operating at abeam voltage of 15 kV.

3. Experimental results and discussion

The characteristic surface aspect of a coir fiberobserved by SEM is shown in Fig. 2. As reported previously[10,13], one should notice that the fiber surface is coveredwith protrusions and with voided areas left by detached

Fig. 1. Aspect of the coir fibers used: (a) tangled fibers and (b) pressed

mat.

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protruded material. In principle, and similar to what wasobserved for piassaba fibers [14], these morphologicalaspects can facilitate the resin impregnation onto thefiber. Fig. 3 shows an EDS spectrum performed at thefiber’s surface. The spectrum reveals, besides the ob-viously carbon and oxygen common to any organic matter,the presence of calcium and other alkaline and alkalineearth elements. Calcium was associated with the protru-sions shown in Fig. 2. This result was surprising, sincesilicon-rich protrusions were identified in a previous work[13]. It is suggested that this marked difference could bedue to differences in the soil where coconuts were raised.A detailed analysis needs to be carried out on this specifictopic. The gold peaks in the spectrum correspond to thesputtered metal used to avoid charging at the microscopechamber during the analysis.

Table 1 presents the average flexural strength andcorresponding standard deviation obtained in the three-point bend tests for the two coir fiber arrangements,tangled or mat, polyester composites cured at the twodifferent molding pressures. As a first comment, oneshould say that composites with less than 50 wt% of fiberswere found to be stiff and relatively hard, while thosewith more than 50 wt% were soft and deformable. There-fore, with respect to the mechanical behavior, thecomposites manufactured act as two completely distinctmaterials. Up to 50 wt% of coir fibers, the manufacturedcomposites are rigid, structural-like, materials. By con-trast, above this percentage, the polyester resin does notproperly impregnate all fibers, even for a moldingpressure of 5.2 MPa. As a consequence, the materialbecomes flexible and easy to bend, performing likebinderless agglomerates [15].

Fig. 4 shows the strength variation with the amount ofcoir fibers for the composites fabricated at the twomolding pressure levels. In these graphs, obtained fromthe data in Table 1, it is important to note the followingpoints. First, for both types of coir fiber and differentcompaction pressures, the strength tends to decrease withthe amount of fiber. This reveals that the randomly

oriented coir fibers are not reinforcing the polyestermatrix at all. Previous data showing the non-reinforcingbehavior of coir fibers in composites submitted to bendingdo not show a steady decrease of the flexural strength asobserved here. The results obtained by Hill and coworkers[12] present a maximum around 50 wt%. Since in thepresent work a smaller span-to-depth ratio (S/d ¼ 9) wasused in relation to that of Hill and coworkers (S/d ¼ 16),and knowing that the smaller the span-to-depth ratio thegreater the contribution of the shear stress, one couldassert that the results obtained are not directly compar-able. The main conclusion of both works, however, is thenon-reinforcing behavior of coir fibers. In fact, thereinforcing behavior of coir fibers in polyester matrix isexpected to be minimized due to the low modulus of thecoir fiber. Values as low as 4.7 GPa are reported [12], andfor an efficient partition of the load applied to thecomposite between the matrix and the fibers, the ratioof the fibers’ elastic modulus to that of the matrix must be

Fig. 2. Surface morphology of coir fibers, showing the characteristic

array of protrusions (’) found at the surface of the fibers.

Fig. 3. EDS spectrum of the fiber’s surface.

Table 1Flexural strength of the coir fiber– polyester composites

Weight % of

coir fiber

Molding pressure

Pressed mat Tangled mass

2.6 MPa 5.2 MPa 2.6 MPa 5.2 MPa

10 25.775.2 31.276.7 29.176.8 32.873.8

20 18.973.6 22.671.2 28.272.1 29.573.5

30 14.574.5 21.473.5 22.579.9 24.773.3

40 9.671.1 11.473.3 20.775.1 23.975.9

50 6.070.9 11.971.5 15.577.9 21.178.4

60 4.371.7 5.971.2 6.775.1 14.375.8

70 3.071.2 4.672.6 5.473.5 8.873.6

80 0.970.6 1.070.4 3.672.2 6.173.0

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maximized [16]. Fig. 5 shows that for fiber/matrix modulusratio close to 1, as would be the case for the compositesanalyzed here, only a fraction of the load applied to thecomposite (Pc) will be carried by the fibers (Pf). Namely,

0:1pðPf =PcÞp0:7

for volume fractions of fiber ranging from 0.1 to 0.7.Therefore, the matrix is always under high loading andfailures under low stress levels are to be expected.

Another point is that the experimental results have arelatively high dispersion, as given by its standarddeviation. This is a consequence of the intrinsic variabilityfound on natural fibers that ranges from their nonuniformcross-section to their mechanical properties. A furtherpoint is that, for both types of fiber used, there is atendency for composites cured at higher molding pressureto present corresponding higher strength values. This isdue to the more effective impregnation of the fibers by theresin, which certainly occurred as a higher pressure isapplied during the matrix setting. However, the curves forboth levels of molding pressure, Fig. 4a and b, fallapproximately within the statistical error related to the

standard deviation. Consequently, one cannot be sure thatfor the molding pressures used in this work pressure hasan actual influence, but just a tendency to improve thecomposite strength. In fact, the molding pressure wasobserved to have only a secondary effect on the flexuralmechanical behavior for piassava–polyester composites[17] and chopped bagasse–polyester composites [18].

In terms of practical interest, the coir fiber compositesmay be regarded as valid alternatives to replace someconventional materials nowadays used by the buildingindustry, and also as furniture. For example, the rigidcomposites with less than 50 wt% of coir fibers can betailored to have tensile strength above 10 MPa, Table 1,which is higher than that of a low-density wood particleboard with 5.5–9.7 MPa [19]. Composites with amounts ofcoir fibers higher than 50 wt%, by contrast, are flexible andcould be used in applications where structural resistanceis not of importance. In fact, and in spite of their relativelylow strength, Table 1, these composites are stronger thangypsum board [19] and can be used in panels or ceilings.Moreover, the fact that coir fiber composites are imper-vious to humidity represents a clear advantage incomparison with the relatively brittle gypsum board,which deteriorates in contact with water.

4. Conclusions

From the experimental results obtained, the followingconclusions can be made:

� Random oriented coir fiber–polyester composites arelow-strength materials, but can be designed to have aset of flexural strengths that enable their use as non-structural building elements.� The lack of an efficient reinforcement by coir fibers is

attributed to their low modulus of elasticity, incomparison with that of the bare polyester resin.

10

Molding Pressure

0

10

20

30

40

2.6 MPa5.2 MPa

TANGLED MASS

Flex

ural

Rup

ture

Stre

ngth

(MP

a)

Amount of Coir Fiber (wt.%)

PRESSED MAT

0

10

20

30

40

Molding Pressure2.6 MPa5.2 MPa

Flex

ural

Rup

ture

Stre

ngth

(MP

a)

Amount of Coir Fiber (wt.%)

80706050403020

10 80706050403020

Fig. 4. Variation of the flexural strength with the mass fraction of coir

fibers and molding pressure. (a) tangled fibers; (b) pressed mat.

1 50 100Ef / Em

0

1

0.5

Vf = 0.1

Vf = 0.3 0.5

0.7

Pf /

Pc

Fig. 5. Variation of the ratio of the load carried out by fibers with respect

to the load applied to the composite as a function of the elastic moduli

ratio and volume fraction of fibers.

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� With the fabrication route used, two different productswere obtained, namely: rigid composites, for fiberloading less than 50% wt, and agglomerates, when thefiber loading was higher than 50% wt.

Acknowledgment

The authors acknowledge the financial support fromthe Brazilian Agency CNPq.

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